I would recommend you use IDRISI. Go to www.Clarklabs.org and let them know what you are doing and ask what kind of a deal they can provide for your use. It might not be free, but it may be for this use.
Also, for the Department you are doing this for, if it is within a University or government, the cost for Idrisi is very reasonable. This is software created under a U.N. charter. It runs on Windows, is relatively easy to use and can import/export to most other GIS systems including ESRI.
Try it, you'll like it.